docs: streamline landing page hero and install flow

- Remove IDENTITY.md system-prompt slide from hero carousel (avoids
  drift with the actual default identity) and renumber remaining slides
- Remove 'What's new in v1.15' section and its nav entry
- Make one-line binary install the default recommendation; target
  ~/.local/bin (no sudo on stock macOS) and drop curl -s so failures
  are visible; go install kept as from-source alternative
- Remove hero install block and Get Started button; the Docker +
  Telegram carousel is now the primary hero call
- Fix install-block CSS: wrap at word boundaries, keep buttons as
  compact pills, generalize green code color

Author: jkyberneees